1. Field of the Invention
The invention described herein relates to communications systems, and more particularly to management of logical channels.
2. Background Art
In systems that follow the Data Over Cable Service Interface Specification (DOCSIS), there is an Upstream Channel Descriptor (UCD) message that is received by cable modems (CMs) in the downstream, i.e., in communications from a headend to a CM. The UCD describes the physical channel parameters of an upstream channel, i.e., a channel used to convey messages from a CM to the headend. The UCD identifies an upstream channel by an Upstream Channel Identifier (UCID) that is unique within a media access control (MAC) domain. A MAC domain is a collection of downstream and upstream channels that is managed under one MAC management entity and that is synchronized under the same timebase. In DOCSIS 1.0, 1.1, and 2.0, after a CM's operation on a particular upstream is determined, the CM only needs to examine UCD messages that contain the UCID of that channel for any changes to the parameters. The CM need not examine messages that have UCIDs for other channels. A change of parameters in the UCD is indicated by an incrementing of the Configuration Change Count in the UCD.
The UCID of the upstream channel also plays a role in the downstream MAP message. A MAP message defines the time and duration of various upstream transmission opportunities on an upstream channel. A MAP message contains a field that has the UCID corresponding to the upstream channel that it is describing. Thus, a CM only needs to look at a MAP message that contains the UCID of the upstream channel that it is using, and need not examine messages that have UCIDs for other channels.
In DOCSIS 2.0, the concept of logical upstream channels was introduced. Multiple logical channels can operate within the same upstream spectrum and are allocated different regions of time in Time Division Multiplex fashion. In particular, DOCSIS 2.0 defined two logical channels to be supported within one physical channel: one logical channel for TDMA transmission and another logical channel for synchronous-code division multiple access (S-CDMA) transmission. Each logical channel is described by its own UCD, and each UCD for a logical channel contains a unique UCID. A CM that operates on one logical channel ignores UCDs for other logical channels that operate in the same spectrum. The CM has no need to understand the existence of other logical channels. The cable modem termination system (CMTS) scheduler keeps track of upstream transmit opportunities that are provided for each of the logical channels as described by the individual MAP messages for each logical channel.
In DOCSIS 3.0, a new type of UCD has been introduced for backwards compatibility reasons. Pre-3.0 DOCSIS cable modems (CMs) will not use this new type of UCD. However, it would be advantageous to be able to support multiple types of CMs, e.g., CMs operating under different communications standards, like DOCSIS 3.0, DOCSIS 2.0, or a version of DOCSIS 1.0 (referred to herein as DOCSIS 1.x) on the same upstream frequency channel without using multiple logical channels. Logical channels can be used for various purposes and therefore allocating them just to separate out 3.0 from 2.0 from 1.x upstream CM communications is not the best use of these logical channels. There is a need, therefore, for a system in which CMs of different types can send upstream communications without having to allocate an upstream logical channel for each CM type.